1. Field of the Invention
The present invention relates to a signal processing circuit for acquiring a ratiometric output, for instance, and more particularly, to such a signal processing circuit that can be applied to a thermal flowmeter or the like for detecting the flow rate of fluid (e.g., the amount of intake air used for a fuel control system of an internal combustion engine) by using a thermal resistance.
2. Description of the Related Art
In general, in a fuel control system of an internal combustion engine, a thermal flowmeter is used to detect the amount of intake air. As an output method for a signal processing circuit applied to such a thermal flowmeter, there have been known various methods such as a voltage output, a current output, a frequency output, a ratiometric output, and so on.
Here, in the case of a signal processing circuit using a method of generating a voltage output and a current output, if there exists an error in a reference power supply connected to a fuel control system when a flow rate signal is AD converted in the fuel control system, a flow rate error will occur, so a reference power supply of high precision is required so as to avoid such a situation.
On the other hand, a signal processing circuit using a method of generating a frequency output or a ratiometric output is known as an output method that does not need such a reference power supply of high precision. In a known signal processing circuit using a ratiometric output, a flow rate signal is once AD converted in a thermal flowmeter and thereafter is DA converted, based on a sensor reference voltage supplied from a fuel control system, into a signal, which is output from the thermal flowmeter.
In addition, as another known signal processing circuit using a ratiometric output method, there has been proposed one in which a peak value of a PWM signal generated in accordance with a flow rate signal is made as a reference voltage supplied from a fuel control system so as to smoothen the PWM signal to acquire a ratiometric output signal (see, for instance, a first patent document: Japanese patent application laid-open No. H11-160121).
In the above-mentioned known signal processing circuits, in order to reduce the influence of digital errors generated when AD conversion (or DA conversion) is carried out to acquire a ratiometric output, it is necessary to set the resolution of an AD conversion circuit (or DA conversion circuit) to a sufficiently fine value, thus posing a problem that the AD (or DA) conversion circuit becomes a large scale. Moreover, in case where a circuit configuration as described in the first patent document is applied, there arises another problem that a response delay in the ratiometric output will be caused upon smoothening the PWM signal corresponding to the flow rate signal.